In a full-duplexed communication system, both the transmitted and received signals are superposed at nearly all circuit nodes. Circuit techniques are employed to eliminate the transmitted signal from the receiver signal path to avoid corrupting the signal-to-noise ratio associated with the received signal. When the signals occupy different portions of the frequency spectrum, this can be achieved by using filters. If, however, their spectra overlap, a circuit commonly referred to as a hybrid must be employed.
Hybrids can be comprised of passive circuits. Typical coupling between the subscriber loop (transmission line) and the modem uses transformers. Transformers provide several advantages including signal gain and isolation, but require the use of inductors in a passive hybrid circuit. This may be problematic since the required inductors undesirably increase the hybrid parts count as well as the area of the modem printed circuit board that employs the hybrid.
In a typical Asymmetric Digital Subscriber Line (ADSL) system, a hybrid circuit is used within the central office modem (in parallel with the subscriber loop interface circuit) to separate transmit signal from the receive signal path. The hybrid circuit characteristics are typically designed to match the transfer function from the transmitter output to the receiver input. The degree to which it achieves this matching dictates the efficacy of the hybrid circuit. A well-matched hybrid significantly attenuates the amount of transmit signal that exists in the receive signal path. This transmit signal, if not sufficiently attenuated, reduces the dynamic range of the receive channel and degrades modem performance.
Central office ADSL modem performance (most specifically upstream data rate) is a strong function of hybrid performance. At the central office, usually the transmit signal is strong and the receive signal is weak. Loop characteristics can vary and hybrid performance typically degrades with varying loop characteristics, such as the loop impedance. The transfer function from the transmitter output to the receiver input varies with loop impedance. This implies that the required hybrid characteristics also vary with the subscriber loop. Hybrid circuits in the prior art are fixed circuits without the ability to adapt to changing requirements. This results in degraded hybrid performance and inconsistent data rates when a hybrid is deployed on a wide range of loops.